1. Field of the Invention
The present invention relates to a sputtering target and a method for manufacturing the sputtering target. Further, the present invention relates to a method for manufacturing a semiconductor device with the use of the sputtering target. In this specification, a semiconductor device means a general device which can function by utilizing semiconductor characteristics, and an electro-optic device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
A transistor formed over a flat plate such as a glass substrate, which is typically used in a liquid crystal display device, is generally formed using a semiconductor material such as amorphous silicon or polycrystalline silicon. A transistor manufactured using amorphous silicon has low field effect mobility, but can be formed over a larger glass substrate. In contrast, a transistor manufactured using polycrystalline silicon has high field effect mobility, but needs a crystallization step such as laser annealing and is not always suitable for a larger glass substrate.
Thus, a technique in which a transistor is manufactured using an oxide semiconductor as a semiconductor material and applied to an electronic device or an optical device has attracted attention. For example, Patent Document 1 and Patent Document 2 disclose techniques in which a transistor is formed using zinc oxide or an In—Ga—Zn—O-based compound as a semiconductor material and such a transistor is used as a switching element or the like of an image display device.
Note that in this specification, an M1-M2-M3-O-based compound (M1, M2, and M3 are metal elements) means a compound containing oxygen (i.e., an oxide) whose main components are the metal elements M1, M2, and M3. At this time, there is no particular limitation on the ratios of M1, M2, M3, and O unless otherwise specified. Further, the compound may contain an element other than M1, M2, M3, and O as long as the element is not contained as a main component. Similarly, an M1-M2-M3-O—N-based compound means a compound containing oxygen and nitrogen (i.e., an oxynitride) whose main components are the metal elements M1, M2, and M3. There is no particular limitation on the ratios of M1, M2, M3, O, and N. Definitions of a metal element, a main component, an oxide, and an oxynitride in this specification will be described later.
A transistor in which a channel formation region (also referred to as a channel region) is provided in an oxide semiconductor can have higher field effect mobility than a transistor using amorphous silicon. An oxide semiconductor film can be formed at a relatively low temperature by sputtering or the like, and a manufacturing process of a transistor using an oxide semiconductor is simpler than that of a transistor using polycrystalline silicon.
Transistors which are formed using such an oxide semiconductor over a glass substrate, a plastic substrate, or the like are expected to be applied to display devices such as a liquid crystal display, an electroluminescent display (also referred to as an EL display), and electronic paper (see Non-Patent Document 1).
It has been found that a crystal structure of a crystallized In—Ga—Zn—O-based compound is an YbFe2O4-type structure, an Yb2Fe3O7-type structure, or a derivative structure thereof (see Non-Patent Document 2). It has been found that a material (an In—Ga—Zn—O—N-based compound) which is formed by substituting nitrogen for 7 atomic % or more of oxygen in an In—Ga—Zn—O-based compound and is crystallized has a wurtzite crystal structure (see Patent Document 3 and Patent Document 4).
Since an In—Ga—Zn—O—N-based compound has higher conductivity than an In—Ga—Zn—O-based compound, the use of the In—Ga—Zn—O—N-based compound as a material of source and drain electrodes or the like of a transistor has been proposed (see Patent Document 5).